While the magnetizable coatings of virtually all magnetic recording media now in use comprise magnetizable particles dispersed in organic binder, the amount of information that can be recorded on any such medium is reaching the theoretical limit. It is known that information can be recorded more compactly on metallic thin-film magnetic recording media, but such media are highly susceptible to abrasion when moved across a magnetic recording head. Hence, such media require some sort of protective covering which should also protect the metallic thin-film coating against corrosion upon exposure to air.
U.S. Pat. Nos. 2,644,787 (Bonn et al.), 3,138,479 (Foley), 3,350,180 (Croll), and 4,210,946 (Iwasaki et al.) disclose various types of metallic thin-film magnetic recording media. U.S. Pat. No. 3,973,072 (Anderson) discloses a nonmetallic thin-film magnetic recording medium for which a protective covering would also be especially useful. Unlike particulate magnetizable coatings, thin-film coatings do not have pores into which conventional lubricants may be adsorbed and hence they are more easily moved by or transferred to recording heads.
U.S. Pat. No. 3,531,322 (Kefalos et al.) discloses a protective covering for metallic, thin-film magnetic recording media which it calls a "pseudo-film". This "pseudo-film" is similar in metallurgical properties to the magnetic layer, e.g., a nonmagnetic Ni-alloy is provided as a protective covering over a magnetic Ni-alloy. A level of hardness approximating that of the magnetic layer is described as providing a two- or threefold increase in useful life, as defined by a drop in readout signal of no more than a few percent. The "pseudo-film" is also said to offer an improvement in corrosion resistance. However, hard protective coverings cause an undesirable degree of wear of the recording head. If of sufficient thickness to be effective, they may produce an undesirably large spacing between the recording head and the magnetic coating. Also, if applied as disclosed in the patent, the hard protective coverings may not have sufficiently uniform thicknesses to insure recording uniformity over the entire magnetic coating.
Lubricants such as are commonly used on particulate magnetizable coatings have also been used in efforts to prevent damage to metallic thin-film magnetic coatings. Among a wide variety of lubricants developed for particulate magnetic recording media are fluorinated polyethers disclosed in U.S. Pat. Nos. 4,268,556 (Pedrotty) and 4,337,139 (Schaefer et al.). The paragraph bridging columns 6 and 7 of the former discloses that such lubricants were applied to the nonmetallic thin-film magnetic recording coating of U.S. Pat. No. 3,973,072 (Anderson). However, it is believed that no material which has been suggested as a magnetic recording lubricant would adequately protect metallic thin-film magnetic recording coatings against abrasion, much less effectively protect against corrosion from exposure to air.
While thin-film magnetic recording media involve special problems as discussed above, there is a continuing need for improving the durability of particulate-type magnetic recording coatings, especially for stop-action use in video recorders.